Introduction

Functional input with format strings

The module Scanf provides formatted input functions or scanners.

The formatted input functions can read from any kind of input, including
strings, files, or anything that can return characters. The more general
source of characters is named a formatted input channel (or scanning buffer) and has type Scanf.Scanning.in_channel. The more general
formatted input function reads from any scanning buffer and is named
bscanf.

Generally speaking, the formatted input functions have 3 arguments:

the first argument is a source of characters for the input,

the second argument is a format string that specifies the values to
read,

the third argument is a receiver function that is applied to the
values read.

if we use stdin as the source of characters (Scanf.Scanning.stdin is
the predefined formatted input channel that reads from standard input),

if we define the receiver f as let f x = x + 1,

then bscanf Scanning.stdin "%d" f reads an integer n from the
standard input and returns f n (that is n + 1). Thus, if we
evaluate bscanf stdin "%d" f, and then enter 41 at the
keyboard, we get 42 as the final result.

Formatted input as a functional feature

The OCaml scanning facility is reminiscent of the corresponding C feature.
However, it is also largely different, simpler, and yet more powerful:
the formatted input functions are higher-order functionals and the
parameter passing mechanism is just the regular function application not
the variable assignment based mechanism which is typical for formatted
input in imperative languages; the OCaml format strings also feature
useful additions to easily define complex tokens; as expected within a
functional programming language, the formatted input functions also
support polymorphism, in particular arbitrary interaction with
polymorphic user-defined scanners. Furthermore, the OCaml formatted input
facility is fully type-checked at compile time.

Formatted input channel

Type of formatted input functions

The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner
is the type of a formatted input function that reads from some
formatted input channel according to some format string; more
precisely, if scan is some formatted input function, then scan
ic fmt f applies f to the arguments specified by the format
string fmt, when scan has read those arguments from the
formatted input channel ic.

For instance, the scanf function below has type ('a, 'b, 'c, 'd)
scanner, since it is a formatted input function that reads from
Scanning.stdin: scanf fmt f applies f to the arguments specified by
fmt, reading those arguments from Pervasives.stdin as expected.

If the format fmt has some %r indications, the corresponding input
functions must be provided before the receiver f argument. For
instance, if read_elem is an input function for values of type t,
then bscanf ic "%r;" read_elem f reads a value v of type t followed
by a ';' character, and returns f v.Since 3.10.0

exception Scan_failureofstring

The exception that formatted input functions raise when the input cannot be
read according to the given format.

The general formatted input function

bscanf ic fmt r1 ... rN f reads arguments for the function f, from the
formatted input channel ic, according to the format string fmt, and
applies f to these values.
The result of this call to f is returned as the result of the entire
bscanf call.
For instance, if f is the function fun s i -> i + 1, then
Scanf.sscanf "x= 1""%s = %i" f returns 2.

Arguments r1 to rN are user-defined input functions that read the
argument corresponding to a %r conversion.

Format string description

The format is a character string which contains three types of
objects:

As mentioned above, a plain character in the format string is just
matched with the next character of the input; however, two characters are
special exceptions to this rule: the space character (' ' or ASCII code
32) and the line feed character ('\n' or ASCII code 10).
A space does not match a single space character, but any amount of
``whitespace'' in the input. More precisely, a space inside the format
string matches any number of tab, space, line feed and carriage
return characters. Similarly, a line feed character in the format string
matches either a single line feed or a carriage return followed by a line
feed.

Matching any amount of whitespace, a space in the format string
also matches no amount of whitespace at all; hence, the call bscanf ib
"Price = %d $" (fun p -> p) succeeds and returns 1 when reading an
input with various whitespace in it, such as Price = 1 $,
Price = 1 $, or even Price=1$.

Conversion specifications in format strings

Conversion specifications consist in the % character, followed by
an optional flag, an optional field width, and followed by one or
two conversion characters. The conversion characters and their
meanings are:

c: reads a single character. To test the current input character
without reading it, specify a null field width, i.e. use
specification %0c. Raise Invalid_argument, if the field width
specification is greater than 1.

C: reads a single delimited character (delimiters and special
escaped characters follow the lexical conventions of OCaml).

f, e, E, g, G: reads an optionally signed
floating-point number in decimal notation, in the style dddd.ddd
e/E+-dd.

F: reads a floating point number according to the lexical
conventions of OCaml (hence the decimal point is mandatory if the
exponent part is not mentioned).

B: reads a boolean argument (true or false).

b: reads a boolean argument (for backward compatibility; do not use
in new programs).

ld, li, lu, lx, lX, lo: reads an int32 argument to
the format specified by the second letter for regular integers.

nd, ni, nu, nx, nX, no: reads a nativeint argument to
the format specified by the second letter for regular integers.

Ld, Li, Lu, Lx, LX, Lo: reads an int64 argument to
the format specified by the second letter for regular integers.

[ range ]: reads characters that matches one of the characters
mentioned in the range of characters range (or not mentioned in
it, if the range starts with ^). Reads a string that can be
empty, if the next input character does not match the range. The set of
characters from c1 to c2 (inclusively) is denoted by c1-c2.
Hence, %[0-9] returns a string representing a decimal number
or an empty string if no decimal digit is found; similarly,
%[\\048-\\057\\065-\\070] returns a string of hexadecimal digits.
If a closing bracket appears in a range, it must occur as the
first character of the range (or just after the ^ in case of
range negation); hence []] matches a ] character and
[^]] matches any character that is not ].
Use %% and %@ to include a % or a @ in a range.

r: user-defined reader. Takes the next ri formatted input
function and applies it to the scanning buffer ib to read the
next argument. The input function ri must therefore have type
Scanning.in_channel ->'a and the argument read has type 'a.

{ fmt %}: reads a format string argument. The format string
read must have the same type as the format string specification
fmt. For instance, "%{ %i %}" reads any format string that
can read a value of type int; hence, if s is the string
"fmt:\"number is %u\"", then Scanf.sscanf s "fmt: %{%i%}"
succeeds and returns the format string "number is %u".

\( fmt %\): scanning format substitution.
Reads a format string and then goes on scanning with the format string
read, instead of using fmt.
The format string read must have the same type as the format string
specification fmt that it replaces.
For instance, "%( %i %)" reads any format string that can read a value
of type int.
Returns the format string read, and the value read using the format
string read.
Hence, if s is the string "\"%4d\"1234.00", then
Scanf.sscanf s "%(%i%)" (fun fmt i -> fmt, i) evaluates to
("%4d", 1234).
If the special flag _ is used, the conversion discards the
format string read and only returns the value read with the format
string read.
Hence, if s is the string "\"%4d\"1234.00", then
Scanf.sscanf s "%_(%i%)" is simply equivalent to
Scanf.sscanf "1234.00""%4d".

l: returns the number of lines read so far.

n: returns the number of characters read so far.

N or L: returns the number of tokens read so far.

!: matches the end of input condition.

%: matches one % character in the input.

@: matches one @ character in the input.

,: does nothing.

Following the % character that introduces a conversion, there may be
the special flag _: the conversion that follows occurs as usual,
but the resulting value is discarded.
For instance, if f is the function fun i -> i + 1, and s is the
string "x = 1", then Scanf.sscanf s "%_s = %i" f returns 2.

The field width is composed of an optional integer literal
indicating the maximal width of the token to read.
For instance, %6d reads an integer, having at most 6 decimal digits;
%4f reads a float with at most 4 characters; and %8[\\000-\\255]
returns the next 8 characters (or all the characters still available,
if fewer than 8 characters are available in the input).

Notes:

as mentioned above, a %s conversion always succeeds, even if there is
nothing to read in the input: in this case, it simply returns "".

in addition to the relevant digits, '_' characters may appear
inside numbers (this is reminiscent to the usual OCaml lexical
conventions). If stricter scanning is desired, use the range
conversion facility instead of the number conversions.

Scanning indications appear just after the string conversions %s
and %[ range ] to delimit the end of the token. A scanning
indication is introduced by a @ character, followed by some
plain character c. It means that the string token should end
just before the next matching c (which is skipped). If no c
character is encountered, the string token spreads as much as
possible. For instance, "%s@\t" reads a string up to the next
tab character or to the end of input. If a @ character appears
anywhere else in the format string, it is treated as a plain character.

Note:

As usual in format strings, % characters must be escaped using %%
and %@ is equivalent to @; this rule still holds within range
specifications and scanning indications.
For instance, "%s@%%" reads a string up to the next % character.

The scanning indications introduce slight differences in the syntax of
Scanf format strings, compared to those used for the Printf
module. However, the scanning indications are similar to those used in
the Format module; hence, when producing formatted text to be scanned
by !Scanf.bscanf, it is wise to use printing functions from the
Format module (or, if you need to use functions from Printf, banish
or carefully double check the format strings that contain '@'
characters).

Exceptions during scanning

Scanners may raise the following exceptions when the input cannot be read
according to the format string:

Raise Scanf.Scan_failure if the input does not match the format.

Raise Failure if a conversion to a number is not possible.

Raise End_of_file if the end of input is encountered while some more
characters are needed to read the current conversion specification.

Raise Invalid_argument if the format string is invalid.

Note:

as a consequence, scanning a %s conversion never raises exception
End_of_file: if the end of input is reached the conversion succeeds and
simply returns the characters read so far, or "" if none were ever read.

Specialised formatted input functions

Warning: since all formatted input functions operate from a formatted
input channel, be aware that each fscanf invocation will operate with a
formatted input channel reading from the given channel. This extra level
of bufferization can lead to a strange scanning behaviour if you use low
level primitives on the channel (reading characters, seeking the reading
position, and so on).

As a consequence, never mix direct low level reading and high level
scanning from the same regular input channel.

Same as Scanf.bscanf, but takes an additional function argument
ef that is called in case of error: if the scanning process or
some conversion fails, the scanning function aborts and calls the
error handling function ef with the formatted input channel and the
exception that aborted the scanning process as arguments.

Reading format strings from input

bscanf_format ic fmt f reads a format string token from the formatted
input channel ic, according to the given format string fmt, and
applies f to the resulting format string value.
Raise Scan_failure if the format string value read does not have the
same type as fmt.Since 3.09.0

format_from_string s fmt converts a string argument to a format string,
according to the given format string fmt.
Raise Scan_failure if s, considered as a format string, does not
have the same type as fmt.Since 3.10.0

val unescaped : string -> string

Return a copy of the argument with escape sequences, following the
lexical conventions of OCaml, replaced by their corresponding
special characters. If there is no escape sequence in the
argument, still return a copy, contrary to String.escaped.Since 4.00.0